text:
How "full-motion" is a full-motion simulator?
How "full-motion" are simulators?  Do they just simulate the major motions of the airplane in response to the flight controls, or do they also simulate the minor forces of the flight, such as the vibration from the engines?
(Bonus question: if they simulate engine vibration, do they simulate the severe vibration from an engine that's just thrown a fan blade?)

Some of them are realistic enough that they simulate the bumps/vibrations in the pavement while taxiing.

Mark, the simple answer is yes , they include vibrations and other minor effects.

(I was trying to dig up a line of code that literally shows "add a bump" for a screenshot, but I can't find the repo!  Heh)

Full Motion is full in the sense that it can generate cues in all 6 Degrees Of Freedom: fwd/aft, left/right, up/down, pitch, roll, yaw. The term came in use to distinguish between 6-DOF systems and the earlier 3-DOF systems which only produced pitch, roll, heave cues.

Do they just simulate the major motions of the airplane in response to the flight controls...

They simulate the aircraft response by stimulating the sensations felt by the inner ear, making use of its limitations as also mentioned in this answer.

The two side-questions about reproduced vibrations boil down to the frequency response of the motion system: the ability of the system to reproduce commanded vibration signals, as measured at the upper platform with the payload installed. Older hydraulic systems, such as depicted above, had a very limited frequency response - the servo valve opens and oil flows through, creating dynamic flow effects and springiness from the compressibility of the column of oil supporting the payload. Yes, oil is compressible. Maximum perceptible cues up to 5 Hz.

Electric systems have an electric motor at the bottom of the actuator. Electric field creation happens almost instantaneously when commanded and the connection to the upper platform is through the metal screwjacks inside the tubes: frequency response is almost an order of magnitude better than that of hydraulic systems. Maximum perceptible cues up to 30 Hz. Enough for engine vibration reproduction, severity depends on the magnitude of the amplitude.
The issues with reproducing severe vibrations at the motion upper platform are:

The cabin construction must be able to rigidly stay in one place and support the visual projectors without starting to wobble the image that the pilots are looking at - the rigidity of the upper construction must exceed the frequency response of the motion system. We've seen simulator cabins bobbing about like a gelatine pudding when subjected to serious amplitude/frequency combinations.
All instruments, electronics, cabling etc. are subjected to the alarming vibrations as well.

Both issues above are not an issue if the constructional design of the cabin etc. has incorporated the vibration inputs. The alarming vibrations are extremely effective training cues, and can be faithfully reproduced by an electric motion system plus suitable cabin construction.
Another way of producing severe high frequency vibrations are by means of a seat shaker or cabin shaker, used in the olden days of hydraulic motion, particularly with helicopter simulators for reproducing the main rotor vibrations. The seats would be mounted on up/down platforms with a stroke of several centimetres, actuated by a separate hydraulic small actuator.

Your "gelatine pudding" comment made me think of a secondary question. Surely someone has also investigated the question of which pattern of simulator motions will produce the most motion sickness in the most users in the fastest time? Either to simulate pilot incapacitation or to avoid any such motions for user comfort?

@JeffY There might have been research on motion sickness in the general public, I’m not aware of a project using motion bases for that purpose. Our inner ear organs get used to being shaken about, after some time people don’t get motion sick anymore.

It’s crazy how that works. Both acclimatization and age have a lot to do with motion sickness. I know a pilot who started flying two years ago and got sick on every flight. After a year, he was over it enough to fly in mild turbulence. Now, he flies Caravans in Alaska. I, on the other hand, have gone from loving roller coasters and aerobatics, to having to watch what I eat if I plan to be inverted or pull negative Gs for too long.

Very well documented explanation.

The feel and graphics in a Level D FFS are pretty darn realistic. Even the feel of vibrations of the engine and the motion during taxiing is pretty well approximated. The entire gimbaled capsule shakes, vibrates and moves on electric jacks. One unit will seat 6-8 people. And, you had better use seatbelts. One unit will take up a room the size of a business jet hangar.
If you are in the Dallas area or in Florida, check out   http://www.atopjets.com/index.html
They fly B737s and A320/330s.
Tell them Dean from the Dallas June class sent you.
I have not experienced a simulated uncontained engine failure. But, hard landings are pretty realistic.
In contrast, a Redbird full-motion AATD still uses Plane-X 11 software. The graphics and motion are realistic for a video game. It compares to a VR arcade or Disney World ride. The Redbird can spin 360°. But, it has limited up and down motion. The entire system can fit in the master bedroom of your house or apartment.
For full-motion FTD rental, check out
https://inthepattern.com/
Or
https://www.thrustflight.com/
The main difference between the AATD/FTD and the FFS would be the force feedback of the controls and the tactile feel of the avionics.
Here are a couple of YouTube videos of the American Airlines Full-motion Flight Simulator Level D.
The ATOP course is taught by longtime airline pilot and AOPA Flight Training Magazine writer/contributor Wayne Phillips.

Dean, appreciate your experience, but be careful making recommendations/referrals. You'll want to make sure you clearly disclose any affiliation (or lack thereof) as we don't want spam.

No affiliation. I just give praise where praise is due. I’m the sort of person to call an organization’s home office when I get exceptional service. Wayne, the owner/operator at ATOPS did a great job. Shout outs to him. American Airlines was also very warm and welcoming at their training center. Great experience over all.

I rent from Sherman Gardner at In the Pattern and from Nathan Rausch’s group up at Thrust Flight. Both great organizations. They host free FAAST seminars and an occasional cook out on the ramp. KADS is also a great venue for July fireworks. Other than that, I am not affiliated with them, neither.

I believe the RedBird Full-Motion Simulators are running a highly-customized version of Lockheed Martin's Prepar3d software, not X-Plane 11. I was unable to find it mentioned in the spec, but the entry-level (non-full-motion) simulators list it in the specifications: https://simulators.redbirdflight.com/products/jay-velocity#specifications. I think it's been so customized for their full-motion sims that it's just a part of their "Red Bird Navigator" software. I don't think calling it " X-Plane 11" is doing it justice, as great as X-Plane is.

You may be right. I have no citations on the type of software used. In the simulator that I used, it’s graphics looked very similar, no better than X-plane.

The Shuttle Mission Simulator's (SMS) Motion Base simulator sat on a standard hexapod motion base so it had the usual three translational / three rotational axes.
Unusually though it had an additional degree of freedom: the cabin could tilt upwards almost 90 degrees to simulate launches. It used an additional hydraulic actuator (highlighted) for this. This degree of freedom was also used to simulate g-loading.

(Picture from old Link newsletter the Link Log, personal copy).
I worked on the motion software some; it was art as well as science.  We simulated random vibration from the solid rocket boosters, transitioning abruptly to the smooth liquid-fueled ride on the main engines at solid rocket booster separation.Checking some old notes, we also simulated vibration/buffet due to the following reasons: passing Mach 1, reaction control jet firings, flat tires, and "runway rumble".
We were left in the dust though by the amazing Vertical Motion Simulator at Ames, which was the cockpit on a hexapod, mounted on a trolley which moved longitudinally along a beam, which beam could move vertically! Kind of like an x-y plotter stood vertically. That was one amazing facility, and I treasure the memory of visiting it. But then the crew would come back and say, we want the SMS to feel like that...

A "6 degrees of freedom" motion base still has to cheat, to emulate real-world motion.
For example, it can't do several consecutive rotations about one axis, like an aerobatic airplane flying rolls.  I haven't heard of one that can even do a half roll (or half loop) into steady inverted flight.
Translational motion also has its limits.  To simulate sustained forward acceleration as with a long takeoff roll, instead of shoving the cab through the wall and across the street, it must eventually pitch the cab nose-up to push you back against your seat.  As the forward acceleration eases off, the cab slowly pitches level again.  Minimizing the felt effects of this "slosh" (faking translation with rotation) is tricky.
But for conventional within-the-envelope maneuvers, it's good enough that lessons learned in it transfer well to the real world.  That matters more than raw physical fidelity.

Yes you’re absolutely right. The real ride has the whole world to create motion effects, a simulator must limit itself to being in a building.

Left-right, up-down, roll, yaw, and accelerate/decelerate. Faking engine vibration, and more violent yawing from engine failure is pretty standard capability.


text:
Can an airplane engine fire be extinguished without shutting down the engine?
Airplane engines catch fire from time to time (and are falsely reported as being on fire even more often).  In a modern airliner, the "fire handle" shuts down the engine, closes the fuel pipe, and discharges the fire extinguisher.
Occasionally it's desirable to leave the burning engine running because you need the added thrust. For example during WWII B-29 engines had an unfortunate habit of catching fire on takeoff. Standard procedure was to leave the burning engine running until you got enough altitude to safely discard the bomb load and/or bail out.
Is it possible to activate the fire extinguisher without shutting down the engine?

Remember that a correctly functioning jet engine is just a carefully controlled fire. So by suggesting fire extinguishing without shutting down the engine, you're proposing to somehow extinguish the "bad fire" while somehow leaving the "good fire" burning (and pumping large quantities of fuel into it). That does sound quite difficult.

I think you should narrow down (or atleast specify) the type of engine you are referring to. The B-29 from your example had piston engines while most modern aircraft are equipped with turboprop or jet engines, which process fuel in a different way.

The engine is already operating outside of its normal regime. You don't have time or means to determine the exact means of failure. Your think the choices are "shut down the engine" or "assume the engine is otherwise operating normally, other than this fire"? Why do you think that this is a reasonable approach? The engine isn't operating normally. You cannot presume anything about its continued operation (such as not throwing blades through the fuselage) - the engine isn't operating normally

@Damien_The_Unbeliever, "I'm on landing approach.  The left-hand engine is missing its cowling, is leaking fuel, and has been shut down.  The right-hand engine just started trailing heavy smoke.  It sure would be nice if I could activate the right-hand fire extinguisher without needing to perform a dead-stick landing."

@DavidRicherby, many of the engine fires reported on Aviation Herald are of the form "slow leak causes flammable fluid to build up in the engine machinery spaces and eventually ignite".  It's quite possible to extinguish that sort of fire while leaving the engine core running.

I don't know. I'm not a pilot. Given time to assess the situation though, I think I'd rather attempt a dead-stick landing rather than a landing offering somewhere between zero and max thrust with no guarantee on thrust versus throttle settings on any particular engine.

I wanted to add to 757toga's answer.
In addition to being required to "pull" the fire handle before you can discharge the fire suppressant, which cuts off the fuel, hydraulics, electrical generation and pneumatic flow to and from the engine. Even aircraft that do not have fire suppression systems (such as the B-52), require the fire handle to be pulled if you have a fire indication, which shuts off the engine.
To the broader part of your question about an aircraft needing the extra thrust. All multi-engine aircraft are certified to be able to complete a takeoff minus one engine from V1 (decision airspeed). V1 is the cross over speed at which the aircraft could safely reject a takeoff without overrunning the runway or can safely continue the takeoff minus one engine. Therefore, there is no time that an aircraft HAS to have the extra thrust from one failed engine.
An interesting corollary from this requirement, is that the most overpowered aircraft will be twin jets (100% extra power at V1), while an aircraft like the B-52 with 8 engines has much lower power margins (14% extra power at V1).

You're assuming that nothing can cause a problem with more than one engine at a time.  Shortly after asking this question, I came across this incident, where a maintenance error left both engines in questionable condition.

@Mark Full engine failure is such a serious condition that your loss of power is becoming almost marginal. Remember that this means loss of hydraulics, electricity for anything but the big six, often loss of quality radio contact and navigational aids. Of course, during takeoff you need the instruments less and the thrust more, but still, you're going to have many more problems than just the thrust. Therefore, all procedures (maintenance and certification especially) are made in a way that this should not happen.

@Mark Dual engine failure is a different animal that aircraft are not really certified for. Thus, pilot's generally have to do the best they can. In the Incident linked, only one engine was on fire and it was shut down once the fire started. The AAIB did note that the fire may have been prevented if the crew had shut down the engine when the fuel leak was first noticed. The crew made the decision to leave the engine on due to the condition of the LH engine, until a fire actually started then they shut it down. A Fuel Feed engine fire is a serious and immediate threat to the airframe, though.

@Mark Another interesting note from the link incident, the suppression system depends on the nacelle being intact to contain the suppressant. So in this case, being able to deploy the suppressant without shutting off the engine would not have actually suppressed the fire.

The very nature of an engine fire suggests that either fuel or lubricating oil is going where it shouldn't be going. Cutting off the source of fuel and the source of ignition is essential in extinguishing a fire. An extinguishing bottle might put out the fire, but if the engine is still running, then it's getting fuel and producing heat... it will probably just reignite.
The B29 engine fire situation was more a matter of desperation than anything. My great uncle was a B29 pilot, killed when the plane crashed on takeoff due to engine failure. The early B29's were prone to engine fires when the engine was under very heavy load, such as during takeoff. If they were fully loaded with fuel and bombs, they couldn't climb or even remain airborne for long on three engines... this was wartime and certain risks were justified to continue the war effort. Modern airliners can climb after losing an engine, and the B29 could lose engines and remain in the air later in flight after it had burned off a good deal of it's fuel load, or better still, disposed of its bombs.
If a B29 engine caught fire on takeoff, it was kept running to try to gain enough altitude to bail out, only because the alternative was definitely to crash. Typically, though, either the engine failed and the plane went down, or the fire weakened the wing and it came off.
The problem was finally solved when they replaced the carburetor on the R3350 engine with fuel injection, but they lost quite a few crews in the process.

My father was a crew chief on the UH-1H model Hueys and I asked a similar question once. I will give his answer, followed by my understanding.
Disclaimer My dad had a habit of giving partly wrong answers on occasion.
His answer: By throttling down to less than the ignition intake.
My understanding: Reducing fuel intake to less than what is necessary for ignition does not quite shut down all engines, some use batteries to keep the engine going and the fuel simply adds thrust. In either case, extinguishing a fire will reduce the thrust available, however. No way around that.

Internal combustion aircraft engines can't be kept going by battery power (unless you count the starter on the APU, which, as the name suggests, is only for starting, not running.) Also, engines require a massive amount of energy. The aircraft do not have batteries capable of supplying that much current and, even if they did, they'd deplete them in short order. The energy density of batteries is very small compared to that of gasoline or jet fuel. Also, any amount of fuel can catch fire, and, if it couldn't, it wouldn't be able to provide thrust.

@reirab I consider myself corrected, then. As I mentioned, my father wasn't always the best source of information... "Look it up" was one of his favorite phrases.

The main reason of a engine fire is the fuel/hydraulic leakage. You have to cut-off the fuel before fire extinguishing, otherwise the fire would be re-ignited even the fire extinguished by the first move.
Due to the high ventilation in the nacelle, fire extinguishing agent can only maintained its concentration less than 1-2 seconds. after you discharged all the agent, you cannot put out the fire anymore if the engine re-ignited.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is this "string" in front of canopy of F-14
I was just watching scenes from Final Countdown movie and this little stringy bit caught my attention. Anyone knows what it is and what is its purpose?
Here is the video I saw it in.

I saw this but not sure if it is the same thing.

In case somebody doesn't want to search the video for it, the above screenshot is from ~4:14 in the referenced video.

Yaw string, also see Yaw String Image Search and the U-2 has one, along with the AF-9J and AV8.

Ok, cheers, seems like it can be closed as a duplicate then. Thank you

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Regulations regarding taking off across the runway
Many years ago, I read an article in Air & Space in which a bush pilot demonstrated the short-field capabilities of his airplane by taxiing out and then taking off across the width of the runway.  What are the regulations regarding this sort of maneuver -- when ATC clears you to take off from a runway, are you required to take off down the runway, or is this merely customary?

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Airline safety: what happened between 1959 and 1962?
The graph on page 17 (PDF page 18) of this Boeing air safety publication shows the fatal accident rate for commercial jets in the US and Canada dropped from about 40 per million departures in 1959 to 2 per million departures in 1962.  Since then, the rate has declined even further, but nothing as dramatic as the original drop.
What happened during the the 1959-1962 period to so greatly reduce the accident rate?

If I had to hazard a guess I'd say it probably has something to do with "burdensome and intrusive government regulation of aviation" (i.e. the establishment of the FAA in 1958/1959)

I believe the answer has something to do with the introduction of the passenger jet airliner, which first came into service in the late 1950s.  Note please that the original poster is asking for information on "commercial jets" whereas the statistics that others have offered have information on commercial planes in general, including prop planes.  I glanced at the statistics for 1959 presented above and there were only two passenger jets (including one KC-135, which you may discount if you like) that crashed.
Compare this to the statistics presented for 1962.  There were six or seven crashes of Boeing and Douglas jet airliners.
To me this means the differences were in all probablility not statistically significant.  We don't have enough "samples" to reach a valid conclusion.
What you can see is a general explosion in the number of people flying.  Jets really introduced the public to flying.

According to my stats textbook, you're right.  You cannot say with 99% certainty, 95% certainty, or even 90% statistical certainty that the accident rate differed between 1959 and 1962.

No doubt, prima facie what's most pertinent needs to be stated at the outset. There is an element of randomness to all air crashes.
That said, sustained efforts by US and some foreign airlines to identify and eliminate minor problems that generally are common precursors to accidents brought down fatality rate drastically from the '60s onwards.
One of the most common scenarios for a plane crash in the '50s was Controlled Flight Into Terrain(CFIT) referring to aircraft that were piloted into the ground, water, mountains or other terrain. However, improvements in cockpit instruments and jet engines dramatically improved aircraft performance and brought down the number of CFITs in the early '60s.
Look at the number of air crashes in 1959, 1960, 1961, 1962. The number of air crashes in the US fell from around sixteen in 1959 to around four in 1962. (Check above link for exact figures.)
Moreover, every single air crash that was successfully investigated provided data to the OEMs that was used to improve the aircraft. For example, the public inquiry into the Comet airliner disaster in 1954 found out that "Metal Fatigue" was the cause of the accident. The investigators found that a small weakness such as this would quickly deteriorate under pressure, and would rapidly lead to a sudden and general break-up of the fuselage.
http://news.bbc.co.uk/onthisday/hi/dates/stories/october/19/newsid_3112000/3112466.stm
Look at this image below. It reflects the causes of air accidents:

You will notice that some of the major causes of air accidents like Pilot Error and Mechanical Failure have gone down between the '50s and '60s.
http://www.planecrashinfo.com/cause.htm
http://www.thepoliticalindian.com/plane-crash-reasons-since-1950/
Lessons were learnt from past accidents. Consequently, companies like Boeing brought about necessary improvements to the avionics and engines. Airliners also helped pilots hone their skills. A combination of these two factors brought down the number of air crashes.
Even to this day, OEMs and airlines are making effort to further improve flight safety.
One oft-cited example is a discovery in the last decade by US Airways (then US Air) that many of its planes approaching Charlotte/Douglas International Airport in North Carolina were coming in “high and hot” - too fast and at a steep angle. After the investigation into this issue was completed, the FAA changed the approach procedure there and the airport installed a system to guide planes at a proper angle.
http://www.ntsb.gov/safety/safety-recs/recletters/A-13-024.pdf

Interesting, but I don't believe this is specific enough to the small span of years asked for.

The number of air crashes in the US was falling from 1959 to 1962. For example, there were 16 air crashes in 1959, 14 in 1960 , 7 in 1961 and 4 in 1962 ( for exact figures check the links above). So, the accident rate was falling steadily. Nothing spectacular happened between 1959-62.

+1. I add emphasis on crew coordination training  & equipment quality; the days of "copilot, your job is to shut up" are over. And that 2-engine planes are allowed on trans-oceanic routes is testament to engine reliability.

When you are looking at "per capita" style numbers you also have to look at the overall volume in your population. Given that the time period is the general start of the growth of aviation, I'm going to guess that you would find that overall flights (and passengers, miles, etc, etc) increased during the same period. That would make the "per X" number of virtually anything you look at in aviation improve.

The whole point of "per X" statistics is that it doesn't matter what the total is. More flights means more potential for accidents.

@fooot thats not always true when you have massive gains in numbers, as happened in the growth of GA

The graph shows fatal crashes per million departures.  If safety remained the same over the time period in question, increased traffic would result in a smoother graph (randomness would have less effect), but not a reduction in the crash rate.

@JimBeam Huh? It's accident per million departures. If there are 1 of them or 20 of them (bunches of millions), why would that change anything. If you compare accidents per total number of departures, then I see your point, 1mil vs 20mil, the numbers would be expected to do as you say. Also, the presentation is by Boeing and about commercial aviation, how is the growth of GA related?

In a sense, this is the answer to the question "How come we still see a few accidents a year, even though flight safety has increased so much?"


text:
Could the cabin crew open the lavatory door from outside when someone locked the door from the inside?
On a recent flight a passenger locked himself in the lavatory for almost the entire duration of the flight. It was only when the flight was in the final leg of descent that he could be persuaded to come out.
In similar situations, can the cabin crew open the door from the outside if it is locked from the inside?

Related (on Travel): Staying in the toilet during landing

Those doors aren't meant to be super secure anyways, so worst comes to worst they could break it down

@DavidGrinberg Or, as described in the link above, simply remove it from its hinges. Apparently the doors are designed for the cabin crew to be able to easily remove them entirely, should they so desire.

@reirab Thanks that link was relevant. I was looking for some guidelines issued by FAA in this regard, as it may be a potential security situation

Apparently yes. It has been mentioned in a number of places. One of the methods given is pretty simple:

Step 1: Approach locked lavatory
Step 2: Lift "LAVATORY" sign
Step 3: Slide the knob into the unlocked position

Thats all to it, though they may vary with the aircraft.

As @RalphJ points out in the comments, it should go without saying that unless you are cabin crew, you should not be doing this.

CAUTION: Chances are good that if you use this information when you shouldn't, you will be the first person to deplane, because the police will be escorting you off to have a chat with them. "Interfering with flight crew" may figure in the conversation. If the lavatory door needs to be opened, better to ask the flight attendants to do so. This info does NOT need to be more widely available, because it CAN get somebody in deep trouble, with no good purpose served by retelling it.

One would hope that most people would have enough common sense to be aware that opening an occupied public bathroom stall without permission has consequences.

I don't know if it's just me, but I always find with good observation, figuring out how to open the door from the outside is pretty straightforward.

@reirab The frustrating part about common sense is how remarkably un-common it appears to be in each passing generation.

@RalphJ: I am sure, all frequent fliers know this trick, which makes me wonder why the aircraft manufacturers don't employ a better procedure, like a master key (available only to the crew). On long haul flights and late at night, somebody might want to enter a toilet already occupied by a female passenger with a wrong intention, and lack of better security might lead to an unwanted incident.

@aeroalias: You forgot to add Step 4: Cackle because you've just unlocked the bathroom from the outside

Yes, the flight attendants can lock and unlock the lavatories from the cabin. As far as I know, the specific ways that they do this, which probably vary from one aircraft type to another, aren't published by the airlines as a matter of protecting customers' privacy. Bad day for everybody if a deviant was to use such a method to open an occupied lavatory to get a cheap thrill.

It's no secret: I've seen how the attendants do it on pretty much every international flight I've taken.

One might conclude locking any bathroom door is merely a way to indicate the lavatory is occupied. Privacy is a relative concept in this world anyway.

authors: Александр Пучков


text:
Strange facility out west near California; what is this place?
While on a flight to San Francisco from New York, somewhere over California or Nevada, I saw this from my window; what the heck is it?

Do you have any reason to believe this is an aviation facility? Is the only link to aviation the fact that you saw this while in a plane?

Honestly, no, just wasn't sure where to post.

Do you have its coordinates?

@Farhan lol, I was in a passenger plane.

@raffian It could be Area 51, that's why I asked for coordinates. Without more information, I tend to agree with the close vote. Btw, on your flight, you flew towards the East of California, then into Nevada.

Perhaps Travel.SE would've been the best place to ask.

I thought it might be Burning Man!

In case anyone's wondering, it's the Crescent Dunes Solar Energy Project, credit to Trish Ling at Sci-Fi.

http://www.solarreserve.com/

http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=60


text:
What's this red cabin light indicator inside a Boeing 737?
What's this red light indicator inside the cabin, its flush with the sides of the plane. The exact location would be a couple of rows ahead of the overwing exit seats.
This picture was taken before take-off. Forgot to observe the light after take-off. This is not a flight-attendant call indicator.

Was there anything similar on the opposite side of the cabin?

@MichaelKjörling No, there was no light on the opposite side of the cabin, It may have been off, that I cannot confirm. I did look on the other side to see if there was a red light there as well.

Kinda looks like one of the lights for the cabin "mood lighting" is not working correctly. Those side lights can be red, blue, green, etc. but they are normally all the same color. Like this

@TomMcW It wasn't 'mood lighting', it appeared to me more as an indicator, in the sense, I could see a similar indicator type section which is near the lavatory.

It might be there for the emergency exit; but other than I have no idea.

This is not an answer (so not posting it as such), but I seem to recall a question/answer here at some point in the past that may be related.  I apologize but could not find it again.  I believe on some aircraft there are interior markings to indicate to the crew at a glance which window(s) are optimal for looking outside to have a good view of the front of the engines in case a rapid visual inspection is needed.  Could this be some sort of indicator of such, since it appears to be right above a window in approximately the right place to look at the front of the B737 engines?

@Milwrdfan I remember the question you're referring to. It referred to A320, which had some kind of sticker above windows. My first thought went towards that, but I couldn't see a corresponding light on the opposite side of the cabin. Waiting for someone with actual knowledge of a 737 cabin to answer it.

@Firee since the winds are on the same spot on both sides.. and it is not a wide-body, one light would be enough.

xpost : https://aviation.stackexchange.com/questions/39634/what-does-this-black-triangle-placard-on-board-an-a320-denote

@Trevor: I know the question you have linked, I even have a comment there 4 months back.

I found this pertaining to the new interior design of some Boeing737(s), I can only guess the coloring is from one of the mood lighting (Blue or red) has a defect in the connections.https://www.airlines-inform.com/flight-reports/amarchukov-1754.html


text:
What are these light like panels on the overhead bins of Boeing 737?
These light like panels are seen in older Boeing 737s. I've never seen them turn on. Could someone enlighten (no pun intended).

I'd take a wild guess and say they're emergency lighting. There is lighting along the floor, including red lights at the exits, so you can see where to go if the cabin is filled with smoke, but if it's a "simple" power failure on the ground and it's dark outside, these may come on so people can see to walk out of the plane in a non-critical-emergency scenario. Much like the emergency lighting in a building - the batteries are charging while normal power is available, and as soon as that's gone, these come one.

Similar 1 -- 2.

@mins Thanks for those pics, I couldn't get a better one.

They are indeed emergency cabin lighting. See, for instance, this photo of a similarly configured 737 interior.

(Łukasz Stawiarz - JetPhotos.net)

Thanks for the image. Do you have any further info as to how they are operated? Do they turn on automatically or activated by pilot/crew? Do they have a different power source?

They are manually operated or armed by the crew and have their own DC battery system.


text:
What is the overhead instrument panel (pictured) near the front galley area of an A320?
I have noticed this overhead instrument panel. It is convered by a transparent plastic lid with a small hole in the middle. Inside it has lot of switches. What is it supposed to do and who operates the same?
I couldn't find an exact picture, but the below image highlights the area where the panel is:

Image Source


text:
Why don't pushback tugs take the aircraft to the runway?
As we know aircrafts have to turn on their engines to taxi the entire way to the runway, if pushback tugs are used, it would save a lot of ATF and overall costs.
Is there any specific regulation by the FAA preventing the pushback tugs from taking the aircraft to the runway, rather than the taxiway?

Image Source: Wikipedia

Let's not forget that the tugs don't move for free. They likely burn less fuel than an aircraft engine, but they're not fuel-free.

Someone did point out here (it might have been in a comment, as I can't seem to find it quickly) that the long, nice warm-up period of taxiing to the runway was very healthy for the engines, and when one airline tried single-engine taxi, maintenance costs on the other engine (the one not used during taxi) took to the skies.

It will congest the taxiway system when the tug has to back-taxi to the gate to get the next aircraft, so any fuel savings will be eaten up by time costs.

Addendum to my previous comment: Anilv's answer to Why not tow aircraft instead of single-engine taxi? mentions the issue of engine warmup.

Way slower, more congested, more damage risk, many more expensive tugs required.  Taxiing under power is much faster and safer than towing in general (towing is a high risk operation and all sorts of bad things happen when tugs move large aircraft). It would be a mess on a taxiway with a line up waiting to take off with trucks and personnel mingling with ready to fly jets.  Generally, when you are flying you want the towing unit disconnected and gone ASAP.
The only reason you have tugs at gates in the first place is because of the need to back up; they're just there for the pushback.  If someone designed a gate with a pass through design that didn't require backing up and allowed for at at least one engine running, you wouldn't have tugs there at all.

'It would be a mess on a taxiway with a line up waiting to take off with trucks and personnel mingling with ready to fly jets' - I think you should expand on, and highlight this point as it's super key. Can you imagine organising the removal of towing equipment at the Threshold or hold!

In the US, there is no regulation on the use of tugs for moving aircraft.  Runway/taxiway environment might get a bit congested at the busier airports.


text:
What's on the upper deck in the picture (circled) of Cathay Pacific Cargo Boeing 747-8F?
Cathay Pacific Cargo Boeing 747-8F

Image Source
Is that a resting area for pilots or passengers seats?

What's behind those windows is the crew seating and rest area.
There are no passenger seats on those planes, the seats are specifically for the pilots and the people handling the cargo.

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

@CarloFelicione why do you think this is not answering the question?

The original (unedited) answer could possibly have been misinterpreted as re-asking the question, but this seems to be quite clear as an answer now.

Cabin crew and pilot's rest area. Here's a pretty good article that also has videos on the upper deck as well as general cargo plane life Life on the Upper Deck

Since this is quite clearly a cargo aircraft (as evidenced by lack of windows at the passenger deck level), and the OP identifies it as a CP Cargo 747-8F, there probably aren't a lot of cabin crew needing a rest area for this flight. ;)

And considering the article states their Cargo plane has a range of 5000 miles, they typically don't fly more than 8 hours. Which wouldn't really warrant a resting crew either. Maybe its more for cargo handlers that might have more legs to their journey?

I'm pretty sure a primary use is to return other crews between flight legs. Deadheading cabin crew on a passenger flight loses you a fare, doing it on a freighter is basically free.


text:
Are “zero-bag fares” really profitable for airlines?
Any extra weight costs the airline in fuel. So, if none of the passengers carry any extra luggage (be it checked in or cabin luggage), will it lead to lesser fuel for the airline, thus lesser expenses, which would justify the lower  “zero-bag fares”.
Or, do the airlines have to meet some minimum weight criteria anyway to make any feasible fuel savings.

In this answer I calculated the cost of taking a few extra kgs on a Airbus A380 physics.stackexchange.com/a/133893/52188

@ROIMaison : I also read a similar comparison of carrying an extra coke can on every flight. It would cost the airline in excess of $1000 each year

That's mostly a marketing ploy; people that don't have any bags are more likely to go with a airline that has a no-bag fare.
However not having any checked-in luggage means less overhead in terms of baggage handling as it's a bag less to load and keep track of. It also means that if you got lost and didn't make it to the gate in time they don't have to offload any luggage but can simply pushback as soon as the gate closes.
The freed-up space is then available for non-passenger cargo transport like mail.

You might add that carry-on baggage is not checked-in at a counter, and every extra bag which needs to be checked is extra effort and expense for the airline. Without check-in baggage you do a self-service check-in in most cases today, so you do the work which otherwise would be done by ground staff.

Also they get to advertise the zero-bag fare on the billboards and look cheaper even if its not true for most passengers who end up checking in something.

Depending on the flight it sometimes IS true for most passengers, but the mention in this answer of bag handling costs is a big part of it too. Knock that cost off, plus the fuel, and that can add up a lot. Plus as mentioned, replacing it with other more profitable cargo can make the airline more money overall

I have always thought that the price is mostly for luggage handling. But how can you then explain that two airlines share a flight, one allows 23kg luggage and one only 15kg.

One thing this does allow the airline to do is sell the space as freight space. Since every plane out there has a maximum take off weight the airlines need to make sure they can fit People + Bags + Fuel on the plane before hand. If they allow each passenger to bring a 50Lb bag then they need to make sure that they can take off assuming everyone on board brought a bag even if no one brings a bag. If an airline can sell a no baggage ticket then they can turn around and guarantee that to freight hauling to make up the costs.
In some other cases it changes how people use the airline as a mode of transport. A lot of people fly for business and generally would stay the night in their intended destination for what may only be one day of meetings. If an airline offers cheap no bag fares an individual may chose to fly out in the morning and back home in the evening after their business is done in which case they may not need to bring a bag. This may help in having people fly both ways on the same airline (although most round trip tickets are booked on the same airline anyway). It also opens up idea of "daily commuting" on a plane.

This is nothing to do with weight or costs. Using a 'zero bag' price with a surcharge allows them to advertise a flight for less, and many people will take that flight over one for twenty dollars more, without checking or realizing that the 'more expensive' flight gives them a free bag and the 'cheap' one charges forty dollars for a bag. It maximizes the chances that your flight shows up at the top when your flight is ordered by prices on the booking website.
Also, not everybody who takes the zero bag fare takes zero bags. Some will pay $50 for a bag, resulting in more profit for the airline.
The other factor is that a zero bag fare captures the people for whom price is a limiting factor - who will only make the flight if it costs them less than \$100. This gets the airline \$100 for the zero bag fare, which would otherwise have been $0 for an empty seat.

Do you have a case to illustrate this claim? I'd like to see a list where the customer is clearly misguided using this trap.


text:
Would a chain strip built in the airport taxiways, help airlines in saving money and time during taxi?
I am referring to the mechanism used in aircraft carriers, which help in catapulting fighter aircraft. What I had in mind is a milder version of the same concept. It would massively save cost, due to it being operated by electric power, save energy and time of the pilots, less exertion.
Would such a system be feasible?

Related: How does an aircraft taxi? and recent question Why don't large commercial airplanes have electric motors in the wheels?.

FYI @Firee, the mechanism in carriers is not cable based, it's a steam powered piston.

Waste of money. Just do this: http://i.dailymail.co.uk/i/pix/2014/09/29/1411966041992_wps_26_ONLINE_EMBARGO_00_01_BST_.jpg

Airports already have several ways to move aircraft along taxiways

Aircraft engines (least economical but always available)
Tugs (often used for pushback from terminals)
Aircraft wheel-motors (some makers are at least planning for electric taxi)

A system of fixed chains along a complex network of intersecting taxiways would be expensive to install and maintain and might be unreliable (e.g. icing up in cold weather).

See also

Does it make sense towing airplanes to the head of airstrip by external means?

With proper planning, the problem of intersections can be overcome, as in railway lines. Plus handling cold weather problems has already been tackled by aircraft carriers. It might be a big cost in installation, but the speed, accuracy and convenience would provide better returns over the years.

I think that a steam powered catapult would have a lot fewer icing problems than a steel cable/chain sitting in a steel track on the ground where all precipitation will naturally collect.  Also, a catapult isn't a mile long.

@Firee "With proper planning, the problem of intersections can be overcome, as in railway lines." How so? Railway lines can just join; moving chains can't.

On a cable car when necessary, a car releases the cable before an intersection, coasts across, then reattaches.  Given that most airport areas are relatively flat and under positive movement control, I think the other issues with such a system are more problematic than intersections are.

Much as I admire Victorian steampunk technology, the reason 1871 patented cablecars were soon mostly replaced by electric trolley cars and diesel buses still apply (except rarely for climbing steep hills, fleecing tourists, etc). I recall SF cablecars get shut down for 7 months at a time for major overhauls. Doubtless more modern cablecars do better but chains and sprockets (or cables and pulleys) need periodic replacement and someone to regularly go around and apply grease, remove detritus from the chainways/cableways etc.

An airport has taxiways that cross other taxiways and runways, so any chain system would require breaks in the chains at these points. Airplanes would need some sort of motive power to cross these breaks, it's impractical and slow for an airplane to spin up its engines for 20 seconds to cross a break in the chain.
A chain system would also be very expensive to install and run, probably more expensive than the fuel it takes to taxi the airplanes.

In addition to all the 'minor' drawbacks already discussed, there is one huge, basic problem with this idea. What has to be moved is an airplane, weighing less than about 600 tonnes, designed to move, with a manageable rolling resistance. A chain or cable system extensive enough to take its payload everywhere it should go adds many tonnes of metal to move with a lot of friction, that will only grow as water and dirt enter the trench. The whole thing will need enormous horsepower just to get moving even without an aircraft attached.
Compared to a tug, or even compared to an aircraft taxiing under its own power, this will simply need too much energy to be feasible. The only place where this system can make money is in some streets in San Francisco.

One point has been missed in the other fine answers.
On an aircraft carrier, the nose gear of each aircraft has a pull bar installed that links with the catapult sled. This pull bar is manually engaged by a deck crew member for each aircraft that is to launch.
In order to do this at an airport, there would need to be a ground handler stationed at each runway turn-off point to engage the pull bar to get the aircraft moving. I pity the poor souls who would have to stand next to a runway at KBOS or KJFK in the wonderful weather they are expecting this weekend.
It's possible that this system could be automated, but that's another very expensive design element that has to be brought into play, tested, and receive CAA certification.

They don't do it for roughly the same reasons that they don't install such systems in roads.

Expensive modifications would be required to every aircraft of a vast array of aircraft types. This alone would probably mean that you'd never recover the costs over the entire expected life of the system. And getting modifications approved by the relevant regulatory agencies for airworthy aircraft designs is even more of a pain than getting them approved for road-worthy car designs.
The systems themselves would be quite expensive for initial install, especially if they were designed to operate in all weather conditions in which airports can currently operate.
Being able to drive where you want is much more flexible (which is somewhat less of a concern at an airport than on roads, but still a concern.)
Stopping or turning suddenly to avoid a collision (like when you're cleared to taxi across a runway and suddenly realize that there's a Learjet taking off on it) might not be possible or would at least be slower.

In short, there are already cheaper alternatives that don't suffer these drawbacks, such as tugs (manual or automated,) electric motors on the gear, and, of course, aircraft engines, as mentioned in RedGrittyBrick's answer.

Also, note that the modifications to fighter planes (Air Force vs Navy) are fairly significant, as the forces on the plane (and humans inside) are significant when using the arrestor (sp?) cables on aircraft carriers.
And yes, reducing the force is possible, but reducing it so that all existing planes flying could use it may (mostly) eliminate its benefit.

A system to taxi aircraft would still have benefit with no need for excessive accelerations needed on carriers. Aircraft are already moved around with tugs, so that shouldn't be a huge challenge.

While the forces would be far less than a fighter landing on an aircraft carrier, an airplane grabbing a chain strip (like the grip of a cable car grabbing the cable to start moving) would be subject to a sudden acceleration rather than the much more gradual one of a normal taxi.

@ZachLipton Indeed. Even at the low speeds of taxi, accelerating a several-hundred-thousand-pound aircraft from 0 to 15 mph in the space of an inch or two is going to take a lot of force. You'd probably have to have some kind of shock-absorbing system that could increase the acceleration distance in order for this to be remotely feasible.


text:
What units are used by western airliners?
Many different units can be used to designate the same things.
Degrees Celsius va Fahrenheit
Altitude in feets or meters, ...
Sometime different units measure the same thing but have different semantics or measure very similar things:
Speed can be measured in Km/h or Mach number.
Fuel can be measured in Kg or liters.
Some units can be also displayed relative to some other ones (ex we have n min of fuel left)...
What units are typically used in aircraft nowadays.
I'm particularly interested in:

Speeds
Pressure
Altitude
Distance
Weight
Volume
Position

Let's restrict the question to units displayed in the cockpit, and to Airbus or Boeing non military airplanes.

Airspeed, ground speed
Primary unit: km/h. Non-SI unit: knot.
In practise, knot is the main unit. At high levels, the Mach number is used, which is a measurement of speed relative to the local speed of sound.
Pressure
Pascal. Hectopascal is used for altimeter settings. In the USA they use inches of mercury.
Altitude
Primary unit: metres. Non-SI unit: feet.
In practise, feet is the main unit. Whether you say altitude or flight level depends on your reference air pressure.
Distance
Long: nautical mile. Short: metres. Vertical: foot.
Mass
Kilograms.
Volume
Litre or cubic metre.
Position
Latitude and longitude based on WGS84.
The above is according to ICAO Annex 5, which means that it is true for more or less all of the world. A few places prefer to deviate from the standard units, such as the USA, which uses a slightly different system.

What do you mean by "primary unit"? And why do you say "Non-SI" unit? Is it meant to be the opposite? If so, why don't you write that?
Is it written like that in the Annex 5?

Yes, that is how it is written in annex 5.

So what is the unit used to report fuel quantities ? Volume or Mass ?

@Antzi Typically mass. Kilograms or tonnes.

These units are either the common ones in the cockpit or what ATC (under the FAA) expects reporting in. Europe may report differently, but I consider the US in the "west" if you further specify where you are asking about the answer may change.
Speeds: Knots or Miles Per Hour, depends on the aircraft and the ASI installed. Some jets also use mach number.
Pressure: Inches of Mercury usually you can find this in the local ATIS. ATC (if you are entering a controlled area) will give you the altimeter setting using these units. For example you may hear "123AB Cleared to the field, Altimeter 29.91" when getting a Class C clearance
Altitude: Feet (usually 100's). When above 18,000 its usually referenced as a Flight Level. ATC expects reports in Feet when announcing altitude.
Distance: Nautical Miles or Statute Miles
Weight: Pounds
Volume: US Gallons (note this is different from an imperial gallon) for fuel or pounds, depends.
Position: "X miles north of Y Location" (or something similar). GPS locations can be used. When flying IFR or on airway a location identifier may be used.
With modern glass cockpits many of these units may be configurable. These units better reflect the older standard gauges used in the US many of which may still be in service.
This FAA handbook outlines instrument layout and the like, you may want to take a look at it for deeper specs but the units included in it are listed above.

https://en.wikipedia.org/wiki/Western_world

I guess it really depends where you are standing, anything is to the west of anything else if you are willing to travel far enough....

This is very US-centric. In other parts of the world, pressure is measured in hPa, weight in kg or tons, volume in litres. In some countries, altitude is metric, too. And even the distance is easier to find in kilometres than in statue miles. (We are still talking about aviation).

I am aware that it is, however I fly in the US and these are the units I use. I was sure to be clear that I was answering for the FAA. I don't know what is used elsewhere as I don't fly out of the country yet. I did not want to answer something incorrectly about the units used in other parts of the world due to my lack of experience.


text:
What was the mass of the Hindenburg?
What was the mass of a fully fueled Hindenburg (discounting any cargo other than fuel/gaz) ?
I do mean to exclude any lift provided by buoyancy.

You mean the weight of the airship plus fuel without any of the lifting gas?

I mean including the mass of the lifting gas

http://www.u-s-history.com/pages/h1648.html 242 Tons

"What was the mass [...] exclude any lift provided by buoyancy": lift/buoyancy are forces (N), they have no noticeable effects on mass (kg), unless they can propel the aircraft at relativistic velocity.

do you want to include the mass of the hydrogen or not?

Mass is independent of lift. If you took the Hindenburg out into the vacuum of space the mass would remain the same.

Steve, Federico: Yes and yes :)

According to airships.net, LZ-129 Hindenburg had a dead weight of 118,000 kg and carried 58,880 kg of fuel. So that comes to 176,880 kg. Here are the full details:

Actual payload from a Hindenburg flight across the South Atlantic using hydrogen; table from airships.net

You forget the mass of the lifting gas, hydrogen in this case...


text:
What happened to ANA 66 on November 12, 2021?
Looking by the window I saw two planes approaching Haneda.
It stroke me as a bit odd that one of the plane seemed to have stopped descent and might not have intended to land.
I checked flight radar, and sure enough the flight did not landed and went around.
Then, a cart came and inspected the runaway.
It was at around 16:00 local time and the closure seems to have lasted just a few minutes.
What happened on this runway ?

Probably a report of FOD by a preceding aircraft.

Not sure how it is "opinion based". Someone with ATC recording could give an answer. We have none right now and that's... fine.

If you noticed an inspection vehicle on the runway in question, some of the possible reason for performing an inspection are:

previous landed aircraft noticed something on the rwy during landing
previously landed or departed aircraft was noticed to be missing some parts
previously landed or departed aircraft reported technical issues during landing or takeoff
specific to some regions, like Japan, an earthquake of sufficient magnitude was registered near the airport (none were registered within the timeframe you specified)
animals, drones or other "unauthorized entities" were seen in the vicinity of runway

In addition to these "ad-hoc" inspections there are regularily performed inspection, such as friction measurement if weather deems necessary, but these are scheduled, and should never necessitate go-arounds.


text:
What is this tank on the wheel of a Westland Wessex HAS31B?
What is this tank on the wheel of a Westland Wessex HAS31B?

I'm talking about the white tank on the left of the picture. Photo taken in Darwin aviation museum, own work, public domain.

I know the grey cylinder attached to the wheel itself is part of an emergency flotation device - perhaps the two are related?

It's the compressed gas bottle (nitrogen or CO2) for the Wessex's emergency water floatation system. The dome on the right outside the tire is a cover for the actual float (sort of like a heavy duty version of a weather balloon). It operates like a Fire Extinguisher bottle, using an electrically fired explosive squib to penetrate a sealing disc and release the gas to inflate the float. It'll be fired automatically when water is sensed, so if you are forced down on water the floats pop out as the gear enters the water.

How do they stop it from firing when it rains a lot?  If pressure activated, if you fly in death valley a lot?

@SethRobertson, the difference in atmospheric pressure between sea level and the shore of the Dead Sea is equivalent to the difference in water pressure between the surface and 60 centimeters below the surface.

@Mark: that's my point.  It looks like the hull is just above knee level, off the ground (round to 1m) and looks like the wheel is about the full height of the ground clearance.  60cm is probably around the actual hull level.  How far does the helicopter have to sink before it activates?

Pretty sure it requires immersion in water so once armed it would discharge as soon as the water was above the tires. Older systems also sometimes were only manually fired.

An air-filled chamber with a downward pointing "drinking straw" entrance would keep rain out pretty well due to surface tension. However, enough water pressure (pressure above ambient air) would force water into the chamber to be detected. Is this how it works?

Not sure of the sensing mechanism itself. You could just use a float switch. If you root around for websites that make and sell these systems, you might find out exactly how they work.


text:
What is this propeller on the A330 MRRT refueling pod?

Illustration taken from this youtube video
Earlier in the video, it can be seen moving:

Is it to provide mechanical power for the boom operation ?

It’s a ram air turbine which drives a fuel pump to provide pressure to transfer gas through the hose and drogue to the receiving aircraft.

Any source ? Why not use electric power ?

@Antzi from the linked duplicated question, "Pods operate with aircraft power or are self-powered by a Ram Air Turbine.". Also, the same solution is used on some EW pods

I believe it actually powers the hydraulic pump for the reel that runs the hose in and out.


text:
What is this sheet on a Super Étendard?
What is this sheet on a Super Étendard ?

As seen on

Don't worry, it's just one sheet to the wind. Joking aside, the first image very much looks like it's from a game.

That is the port-side ventral airbrake, as can be seen in this diagram:

It's an airbrake. there are both of them, each side of the ventral face. it moves with an hydraulic actuator

Welcome to Aviation.SE!

Well, he did mention the hydraulic actuator.


text:
How is the runway pavement chosen?
Wikipedia list a number of runway pavement. For specific airports, the choice is obvious (e.g. ice, snow). But there are few pavement I cannot figure out how to decide which one use.
Why an airport would choose asphalt over concrete or bitumeous asphalt for the runway pavement? What are the reason for the choice?

Asphalt is cheaper and faster to build, but not as strong.

@JanHudec how much cheaper and faster? what do you mean by "strong"? support heavier aircraft? better absorption of ground movement (e.g. earthquake)? better tolerance to temperature changes?

I am neither builder nor airport manager, so I don't really know how much cheaper asphalt is compared to concrete (and there are various kinds of each), but it is obviously quite significant since concrete is only used for the highest loaded runways and roads. By stronger I mean that it can support higher load, that is heavier aircraft and/or more of them. Asphalt is slowly deformed by both high loads and repeated moderate loads. It is however more easily repaired than concrete.

for the not as strong you can look at this answer: essentially because asphalt isn't rigid it will get rutted and potholed eventually.

@JanHudec asphalt also provides a smoother ride (no cracks between the slabs), and is more porous so it's easier to drain rain water. Disadvantage is that it is more easily damaged in icing conditions.

I am neither builder nor airport manager and can't provide details, but as there is no answer yet, I'll try to summarize the main differences between the materials:

Concrete can support highest loads and lasts long. It is however most expensive to build and when it eventually gets damaged, it can't be patched and has to be rebuilt in large slabs. It also requires stable base, because it is rigid. It is built in slabs with spaces between them that are required due to account for thermal expansions and give somewhat bouncy ride.
Asphalt. There is a big variation between different kinds of asphalt concrete, but it is generally cheaper than concrete, but can't support as big loads and is susceptible to damage by frost. Asphalt is never completely rigid and slowly deforms under load. This allows using it on unstable base, but it means bumps form by prolonged use reducing the total lifetime. It also easily cracks and when water freezes in the cracks, potholes quickly form. They can however be relatively easily patched. It does not require any thermal expansion gaps and provides smooth ride.
Asphalt and concrete can be combined. A concrete with a layer of asphalt on top provides the strength of concrete and smooth ride of asphalt. The asphalt also protects the concrete base from freezing water while itself can be easily patched. This is lately common on roads (where concrete alone is really uncomfortable), not sure how much on runways.
Gravel is simple to build; all you need is some stone from nearby quarry and a steamroller. The main disadvantages are risk of foreign object damage by loose stones thrown up by the wheels and need of regular maintenance. So it is used mainly in remote cold places where concrete would be too expensive and asphalt would not last long in prevailing weather conditions.
Rolled earth with or without grass is even simpler then gravel, used for small airports where big loads are not needed.
When runway is listed as grass, it means it is not even rolled. Usually enough for small airplanes and obviously cheapest.
Other surfaces usually as provided by local conditions (snow, salt etc.)

For every particular airport, the investor evaluates the available technologies whether they can handle the expected traffic (to handle large jets like 747 or A380 concrete or concrete-asphalt combination is preferred), whether they are buildable in given location (the ground has to support the weight too) and how much building them will cost and choose the option that satisfies the requirements and they have money for.

I think that runway surface is chosen because of the intended traffic. For A380, Airbus requested/directed/lobbied many airports to enhance their runways and other facilities. Your answer does highlight the intended aircraft which a runway can handle.

At FRA the both main runways 07C/25C (Center) and 07R/25L (South) are made from asphalt and used by large jets (B747-400, B747-800 and A380). I wouldn't say "only concrete will do".  https://en.wikipedia.org/wiki/Frankfurt_Airport#Runways (german wiki says 18 uses mixed concrete and asphalt)

From an engineering perspective, concrete is modeled as a rigid pavement and asphalt is a flexible pavement.
Concrete is harder and more brittle than asphalt. Concrete generally cannot tolerate significant ground movement without cracking.  The addition of steel reinforcing to concrete can help to mitigate concrete's brittleness to some extent, but Jan is correct in saying that once concrete has cracked or spalled, it cannot be easily repaired.   Generally an entire panel must be completely replaced.
Asphalt is less expensive than concrete to install initially but the surface will degrade over time due to the evaporation of volatile aromatic hydrocarbons so must be maintained on a regular schedule whereas a concrete surface requires no regular maintenance.  Asphalt is a weaker material than concrete but can be made to support equal loads by constructing a thicker pavement.  Asphalt will soften under high temperatures so warm climates can be a problem for asphalt.  Because asphalt is flexible it is a better choice over soils that are prone to movement or over soils that would be too expensive to treat to raise their bearing capacity.
Concrete reflects light better than asphalt so will have better visibility at night.  For highways, asphalt is a quieter surface than concrete although that would not be a consideration for aviation purposes.
The choice of whether to use concrete or asphalt must consider all these variables of initial cost vs. long term maintenance, soil condition, visibility, climate, local budgetary considerations, local availability of materials, etc.  Each installation will be different.
I have supervised drilling cores from the runway and taxiways at San Diego International Airport (KSAN).  It has alternating layers of concrete and asphalt in some locations although the top surface is asphalt.  The total thickness varies but is generally at least 18" thick.  The airport sits on the former outlet to the San Diego River and is only a short distance from the bay, so the sub soils are weak alluvial silts that are saturated a short distance below grade.  Yet the taxiways are mostly concrete.  Go figure.


text:
Is it safe to use a parallel runway's PAPI/VASI?
Many airports have parallel runways but only one runway has a PAPI/VASI. If both runways start at the same point and have similar touch-down points, is it safe to use the parallel runway's lights for the approach and landing?

"If both runways start at the same point and have similar touch-down points", that's a pretty big if :)

Per the AIM (2-1-2 - Visual Glideslope Indicators)

The visual glide path of the PAPI typically provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 SM from the runway threshold.

(Deviations from that are supposed to be noted in the Airport/Facility Directory, for example if the clearance path is offset from the centerline, or theoretically if you could use a parallel PAPI...)
A few miles out you're probably within the 10 degree arc, so the PAPI will be useful, but as you get closer that arc narrows, and as Lnafziger mentioned there might be an obstacle in your way (like the FAA-Standard 50-foot tree right at the threshold line).
Also as abelenky pointed out you're not supposed to be using parallel PAPIs/VASIs for approach guidance - It's a good cross-check, but not a primary reference.

It would also depend on how far apart the runways are.  :)

@Lnafziger Indeed, for a number of reasons. In addition to the arc with a large runway complex there may be a noticeable difference in elevation between parallel runway ends.

Not necessarily.  The PAPI is installed, and the angle is set for the obstacles in the approach path for that specific runway.  There could be obstacles on the parallel that the other PAPI would not give you clearance over because it was never surveyed for that.

It is considered "safe" for a pilot to use all available information to safely conduct a flight.  This does not exclude using other references, such as a parallel PAPI as a means to cross-check.
However, it is not legal to use a parallel PAPI as your primary guidance.   If there were to be an incident, and you were to tell an investigator, "the parallel PAPI indicated that I was on the GS", he'd think you were a bit nutty, and fault you.
If instead, you were able to tell him "I was properly lined up for the Left, and then cross-checked with the PAPI on the Right, which also indicated a stabilized approach on the GS", he might commend you for creative cross-checking.


text:
Are commercial aircraft water tight by design?
If its submerged in water, will water rush inside the cabin? For instances, if its standing in a flooded airport or landed on the water gently.

No, its not a duplicate

Specifically, this answer is relevant to the question (even if it isn't very long or thorough).


text:
What's the minimum cruise speed of modern airliners?
Often when approaching landing, I feel the aircraft slow down considerably, it hasn't landed yet, but kind of hovers around waiting for clearance to land. Thus I was wondering, is there any defined minimum cruise speed for airliners.

It's called the stall speed. Below which the wings won't give enough lift to stay airborne.
Another minimal speed to adhere to is the minimal control speed. Below that the controls surfaces won't be able to counteract suddenly losing an engine.
However slowing down before landing has nothing to do with waiting on their turn/clearance. If they need to wait they go around in a circle instead.
The slowing down is so they touch down with as low speed as possible to limit the braking distance needed on the runway.

The lift a wing is able to generate is proportional to square of speed¹. If the aircraft is moving through the air too slowly, it will stall², pitch down and start to descend rapidly.
So a fixed-wing aircraft³ must move at least some speed, called stall speed or $V_S$. And normally some margin error should be left, so the aircraft never flies slower than about $1.3×V_S$.
What the $V_S$ is depends on the aircraft and the weight. Because lift is needed to balance weight, less weight means less lift is needed and thus less air flow to generate it. For jet airliners the stall speeds may range from around 100 knots when light (~185 km/h, ~115 mph) to maybe 130 knots (~240 km/h, ~150 mph) when loaded⁴.
On the other hand, at altitude below 10,000 ft, maximum speed of 250 knots (~463 km/h, ~288 mph) is usually defined so the pilots have enough time to see each other when flying near the airports under visual flight rules or in case the controller makes a mistake.

¹ It is actually proportional to dynamic pressure, which is proportional to pressure and square of speed, so as the aircraft climbs, the minimum speed increases. Modern airliners fly very high (usually 32,000 to 42,000 ft) where the lower pressure and corresponding lower drag allows flying much faster, but the minimum speed is also higher.
² Lift is proportional to dynamic pressure and angle of attack. At slower speed the wing flies at higher angle of attack, which is how the nose of the aircraft still points up when it is approaching to landing. The wing is able to provide more lift up to critical angle of attack above which it stalls.
³ Rotorcraft (helicopters) create the air flow by rotating their wings instead and therefore don't need any forward speed. In fact they stall when flying too fast instead when the retreating blade is no longer moving aft fast enough.
⁴ These are values for high lift devices (flaps and slats) deployed. With them retracted, the stall speed is higher. The stall speed also depends on the relative wing area of particular aircraft. For example A318, A319, A320 and A321-200 all have the same wing and therefore the same stall speed at the same weight, but they have different size and therefore different typical weights. There is similar difference between say B737-700, B737-800 and B737-900.

Very nice! I like the simple answer with the footnotes to indicate that you've covered all your bases.

A stall won't necessarily result in a nose-down attitude (and of course, recovery usually needs to begin with the pilot pitching the aircraft down in addition to adding power for speed, then pulling back up). Take AF447 as just one well-known example; Koyovis' answer to On Air France 447, what would have been the lowest altitude to initiate recovery after the stall developed? quotes the accident investigation report as specifying a vertical drop of 10912 ft/min and a 16.2 degrees nose-up pitch attitude as the last values recorded by the FDR.

@MichaelKjörling, well, stall will always create a large pitch-down moment. The pilot may, however, have just enough elevator authority and keep the plane stalled. On stable aircraft with normal controls, the large force needed to keep the nose up can be considered a telltale sign of speed decay, but some pilots still managed to stall their planes this way, not realizing what was going on (I've read about at least two such accidents, but don't remember enough to find them now). On AF447 the Airbus alternate law made things worse by auto-trimming all the way nose up as the speed decayed.

The minimum speed at which the aircraft can fly (cruise) is called the stall speed. At this speed, the lift is equal to the aircraft weight.
The lift L is given by,
$L = \frac{1}{2}C_{L} \rho S V^{2}$,
where,
$C_{L}$ is the lift coefficient, $\rho$ is the density at that altitude and $S$ is the wing planform area. During level, unaccelerated flight, the lift is equal to weight.
$L = W = \frac{1}{2}C_{L} \rho S V^{2}$
This gives the stalling speed as,
$V_{s} = \sqrt{\frac{2 W}{\rho C_{L_{max}} S}}$
This is the minimum speed at which the aircraft can fly. Note that it depends on three things: the Weight of the aircraft, the density of the air and the $C_{L_{max}}$ of the wing section.
During landing, two things happen: the weight is lesser (compared to takeoff weight), and the flaps are deployed, which increases the $C_{L_{max}}$, which in turn reduces the minimum speed required for flight. This is the reason you're feeling a reduction in speed.

Source: assets.decodedscience.com

Source: Stall Speed, Prof. Dr. Mustafa Cavcar
In general, aircraft are flown well over the stall speed. If the margin (between flight speed and stall speed) is maintained, then your flight speed will reduce when the stall speed is reduced.

Arguably, an aircraft can cruise at any speed as long is it does not stall. However, sometimes, ATC will notify a flight crew and tell them to maintain a certain airspeed due to traffic or other factors. Keep in mind, stalling is not based on speed as much as it is based on AOA (angle of attack). Angle of attack is the angle that the air meets the wing (in case you did not know). Stalling is when the airflow over a wing becomes broken and turbulent or separates from the wing. To a certain point, a higher angle of attack will grant more lift (that is the point of flaps). However, it can be dangerous since it can also induce a stall.
https://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/StallFormation.svg/350px-StallFormation.svg.png
The above picture displays what I am trying to say.  So as long as critical angle of attack is not reached, an aircraft can technically fly at any speed upon landing (although ATC will often request airplanes to keep airspeed relatively high or low so traffic runs smoothly). The book, "Stick and Rudder" has an enormous section on AOA, among many other things.


text:
Do aircraft manufacturers have the authority to either approve or disapprove the maintenance procedures of its customers?
Having currently read this article in Wikipedia, linked through Reddit, regarding American Airlines Flight 191, which crashed due to maintenance issues.
Inside the article, it says

McDonnell-Douglas, however, "does not have the authority to either
  approve or disapprove the maintenance procedures of its customers."

Is that the scenario in the current aviation industry, or has it changed. Why would airline companies not listen to the manufacturer and carry out non-approved procedures.

Why would airline companies not listen to the manufacturer and carry out non-approved procedures..  You assume that they do?  The statement about McD-D is simply a different way of saying that the authority lies with the certification body, not the manufacturer.  In most cases, the procedures authorised by the certification body are the same but may differ where national law requires it.  Or, put another way, McD-D has no authority to certify something as lawful or not in any given jurisdiction.

The above accident happened because american airlines carried out non-approved maintenance procedures. Please read the linked article.

This is a very broad question (though that fact may not be obvious). Would you be interested in narrowing it by jurisdiction and segment of the aviation industry, for example: FAA Regulations, and Airline Operations?

@JonathanWalters since it involves american airline with american manufacturer I would assume it is faa regulations.

@vasin1987 I prefer not to assume. Moreover, the way this site works specificity is better, especially in clarify the distinction between possible duplicate questions. My request for clarification stands.

The procedure was unapproved by the FAA.  It was therefore illegal.  AA were fined $0.5m.  The type certification was later amended to make this explicit.

@Simon Could you please elaborate?

@JonathanWalters : Is a jurisdiction on this mandatory? For example Boeing supplies its aircrafts to numerous airlines across the globe, so it would be a long list.

@Firee But answering the question as written would require something like this to fully answer since it is asking a question about legal authority. "Well, in the US the FAA does not allow Boeing to do it, in China the xxxx does/does not, in Russia the xxx does/does not", etc, etc.

@Simon The maintenance regs are a little more complicated than that: It's not so much that it wasn't "approved", but rather the fact that AA's shortcut wasn't described in the manufacturer's maintenance manuals for the airframe or engine, and couldn't be broken down into smaller operations using "methods, techniques, and practices acceptable to the administrator" in something like AC 43-13 or the AMT handbook that made it a violation.

@voretaq7 Yep.  I used to work for a "designated engineering authority" of the CAA and UK Ministry of Defence.  The salient point is that McD-D cannot have any authority over the regulatory body in another country, hence why the authority for repair processes lies with the certifier, not the manufacturer.

@Lnafziger: For the sake of good order, let's specify jurisdiction for the question above to US, UK, Europe & Asia.

For N-Registered Planes Under FAA regulations... (this may differ elsewhere in the world)
The short answer is No (although some service bulletins may be legally required), the manufacturer can not legally enforce anything per say, however here's how it plays out.
Most makers are at least in some capacity always testing their aircraft and aircraft procedures (at least the models in service). This may be as simple as inspecting the ones out there and looking at used parts for fatigue and what not. This in turn can lead to the company issuing a Service Bulletin which is basically a document that says: "hey here's something we found, you should look at it". This can apply to a any part of the aircraft as well as inspection and maintenance procedures. However service bulletins are not legally required to be complied with when issued by the company (although many may choose to comply).
On the other hand the Service Bulletins may trigger the FAA to issue an Airworthiness Directive which is a legally enforceable document. In this case the owner/operator of the aircraft is legally required to comply and alter their procedures.
In this case of the accident in question I think this is the AD that was issued as a result. You can find a full list of MD-10 AD's Here.
On to the question of why an airline may not listen. This is most likely a cost related issue. If the airlines insurance does not change and the cost to inspect or possibly replace a part that they don't legally need to replace is simply to high, they wont do so until its legally required. Again since every thing in aviation has two sides, manufactures may have the courtesy to replace the part at no cost to the owner. For example, when Porsche dropped the FAA Type Certification for the 3.2 motor in the Mooney M20PFM leaving all the owners somewhat high and dry, Mooney was kind enough to have the 40 odd aircraft updated with Continental motors (although many were destroyed in a hurricane while under going the change). The crux of it also comes down to the fact that only the government and its various organizations (here in US) can legally impose anything on an individual or company. So while it may be a good idea to heed the advice of the maker you may not be required to do so.
--Edit--
To answer the comment below which is somewhat relevant to the original question. My understanding is that yes the FAR's may require you to refer to the manufacturer procedure but at the same time you only must refer to the procedures required (associated with the current certification of the airframe) for your airframe an SB with out an accompanying AD is a manufacturer procedure but is not legally required. So you may refer to it but you may not have to comply with it. Ill dig into the regulations and see if I can find the letter of the law on it.

I doubt that the FAR's lay out maintenance procedures for every aircraft in detail like the manufacturers do. Don't the FAR's usually just refer to the manufacturers' procedures? Something like "no one may operate an aircraft without performing maintenance as described in manufacturers recommendations." (Just a guess at the language. I don't know the associated FAR's) Thus meaning that although the manufacturer doesn't have legal authority, their published procedures become de facto law.

@TomMcW At least in the UK, that's pretty much what happens.  Some large percentage, let's say 99% of procedures are not specifically mandated but just refer to the manufacturer.  Deviations are made where the manufacturers process is not compatible with UK law, e.g. health and safety legislation, use of certain materials (some materials used in the UK are banned in other countries and vice-versa) and so on. However, the wording is such that the release to service is legally enforceable.  Where not directly mandated, not complying could be an offence. I can't speak for any other jurisdictions.

Short Answer:
Generally, no, but in at least one notable exception, Yes.
Longer Answer:
@Dave has given a good answer that covers much of the industry. However, there is an industry sector for which the answer is, "Yes": FAA certified Special Light Sport Aircraft (S-LSA). These maintenance requirements are found in part 91, not—curiously enough—in part 43.
Firstly, the FAA gives manufacturers of S-LSA the authority to prescribe what must be done for routine maintenance and inspections. This authority is not absolute, as part 43 also applies and the role is also open to "a person acceptable to the FAA".
See 14 CFR 91.327 (b) (1) and (2):

(b) No person may operate an aircraft that has a special airworthiness
  certificate in the light-sport category unless—
(1) The aircraft is maintained by a certificated repairman with a
  light-sport aircraft maintenance rating, an appropriately rated
  mechanic, or an appropriately rated repair station in accordance with
  the applicable provisions of part 43 of this chapter and maintenance
  and inspection procedures developed by the aircraft manufacturer or a
  person acceptable to the FAA;
(2) A condition inspection is performed once every 12 calendar months
  by a certificated repairman (light-sport aircraft) with a maintenance
  rating, an appropriately rated mechanic, or an appropriately rated
  repair station in accordance with inspection procedures developed by
  the aircraft manufacturer or a person acceptable to the FAA;

In addition to this, manufacturers of S-LSA can issue Safety Directives, which are generally mandatory and can be thought of as roughly equivalent to Airworthiness Directives (ADs). Again, this power is not absolute.
See 14 CFR 91.327 (b) (4):

(4) The owner or operator complies with each safety directive
  applicable to the aircraft that corrects an existing unsafe condition.
  In lieu of complying with a safety directive an owner or operator may—
(i) Correct the unsafe condition in a manner different from that
  specified in the safety directive provided the person issuing the
  directive1 concurs with the action; or
(ii) Obtain an FAA waiver from the provisions of the safety directive
  based on a conclusion that the safety directive was issued without
  adhering to the applicable consensus standard;

1 That is, the S-LSA manufacturer.
Furthermore, any repairs or alterations must be approved by the manufacturer, either by the existing manufacturer supplied Maintenance Manual, or on a case by case basis for those falling outside the scope of the Maintenance Manual. Once again, this power is not absolute; the FAA has to retain some authority.
See 14 CFR 91.327 (b) (5) and (6):

(5) Each alteration accomplished after the aircraft's date of
  manufacture meets the applicable and current consensus standard and
  has been authorized by either the manufacturer or a person acceptable
  to the FAA;
(6) Each major alteration to an aircraft product produced under a
  consensus standard is authorized, performed and inspected in
  accordance with maintenance and inspection procedures developed by the
  manufacturer or a person acceptable to the FAA; and

See also this article by D. Martino: Work Books: A flight instructor’s guide
to S-LSA maintenance regulations, which gives a good interpretation of these rules with less legalese.

@Firee This is why I proposed limiting the scope of the question towards the industry sector you highlighted in your example case.


text:
If the cabin temperature controller does not function, does the pilot have to make an emergency landing?
The cabin temperature controller in commercial airlines, controls the cabin temperature. But if for any reason, it does not function, does the pilot have to make an emergency landing, or can he cruise to a lower altitude for normal temperatures?

Probably not, unless the pressure / air renewal is also malfunctioning (systems are related).

At the worst this would be a "pan pan" situation, not an emergency but I can't see a malfunctioning air conditioning being that much of an issue. Typically there are enough bodies on board to keep it relatively warm at any altitude. If the passengers were extremely uncomfortable the pilot may ask to divert but that in-of-itself is not an emergency, and would probably make for a short airline career if the pilot declared an emergency for malfunctioning air temp controls.

I'll use the B757 as an example, on it you would not divert due to temperature control issues. In fact the Master Minimum Equipment List items 21-61-1 and 21-61-2 give permission to dispatch the aircraft with inoperative zone temperature control.
The Quick Reference Handbook (PDF) for the 757 gives the following instructions for failure of zone temperature control.

I also think most airlines and crew think passengers would rather get to where they are going than being comfortable.
As for the emergency landing, aircraft can and do divert all the time without declaring an emergency (Mayday) or urgency (Pan-Pan). So worse case the crew would choose to divert to a suitable airport.

This is just the zone control failing, with packs still providing air of reasonable temperature. If the packs themselves go haywire and start outputting air of unreasonable temperature (too hot is more likely), you've effectively lost them and have to divert. Of course you have two, so it is unlikely both fail unless one was inoperative from the start.

@Jan Hudec I agree if you lost both packs you would most likely divert. The more pressing issue would be the loss of pressurization. The packs do have their own, independent temperature controllers, as well, with an extra back-up controller. I did narrowly interrupt the question to mean just the cabin temperature control, though.